It is important to note that there are two types of diversity in a laboratory facility:
·An HVAC diversity factor, which relates to the thermal characteristics of the facility's envelope, diurnal temperature swings, and occupancy of support spaces.
·A fume hood diversity factor, which relates directly to the demand characteristically placed on the ventilation system. [Rabiah and Welkenbach, 1993; Lentz and Smith, 1989]
The fume hood diversity factor is the percentage that is used from the maximum amount of exhaust air-flow available from the laboratory ventilation system at a moment in time. Therefore, the fume hood diversity factor relates directly to the percentage of fume hood sashes that are open in operating hoods. This diversity factor is also a function of the hood users' work loads, research functions, and work habits. [Varley, 1993]
The fume hood diversity factor has been thought to be associated with the number fume hoods in the facility per total building area. Studies have shown that this assumption is not correct in all cases. The ratio of hoods to laboratory personnel contributes more to the diversity factor. Linear regressions were performed on plots of diversity factors against hood/personnel ratios; the regressions showed a strong correlation between the diversity and the number of hoods per user. "This makes sense, since one would expect hood utilization to increase as more personnel were required to share the available hoods." [Rabiah and Welkenbach, 1993; Hitchings and Shull, 1993]
Naturally the energy engineer must exercise caution before attempting to apply these findings. Extrapolation to facilities with small numbers of people or hoods is not recommended, for example. This procedure does not account for a lower diversity limit that requires a specific exhaust rate fraction of the total be maintained. [Hitchings and Shull, 1993; Rabiah and Welkenbach, 1993]
The lower diversity limit is a minimum value determined by a variety of factors including control set points, envelope and system leakage rates, heating and cooling loads, and indoor air quality standards. Leakage rates around dampers or at duct connections will influence the minimum value. Depending upon the climate, the air required to remove sensible heat gain may limit the diversity factor. [Rabiah and Welkenbach, 1993]
Diversity factor—a case study